1. Field of the Invention
The present invention relates to a phase controller used as a light controller of an incandescent lamp.
2. Description of the Related Art
A phase controller using a bilateral thyristor (hereinafter called TRIAC) is conventionally used as a light controller of an incandescent lamp. Circuit diagrams of typical phase controllers are shown in FIGS. 22A to 22C.
The phase controller has two terminals respectively connected between both terminals of a series circuit of a commercial power source AC and a load such as an incandescent lamp LA. As shown in FIG. 22A, a TRIAC Q1 is connected in parallel with the series circuit of the commercial power source AC and the incandescent lamp LA. A trigger circuit T is configured by a damping time circuit which is a series circuit of a resistor R1, a variable resistor R2 and a capacitor C1, a DIAC Q5 connected between a connection point of the capacitor C1 and the variable resistor R2 and a gate terminal of the TRIAC Q1, a reverse series circuit of two Zener diodes ZD1 and ZD2 connected between both terminals of a series circuit of the variable resistor R2 and the capacitor C1 for maintaining a voltage between the both terminals constant. A phase controller shown in FIG. 22B uses a trigger circuit T in which an SBS (silicon bilateral switch) Q5′ is used instead of the DIAC Q5.
In the conventional phase controllers, when a voltage of the capacitor C1 which is charged through the resistor R1 and the variable resistor R2 reaches to a breakover voltage of the DIAC Q5, or when a voltage applied to a cathode of a Zener diode ZD3 which is connected to a gate of the SBS Q5′ becomes higher than a Zener voltage and a gate current flows into the SBS Q5′, the DIAC Q5 or the SBS Q5′ is turned on. Thus, electric charge in the capacitor C1 is released as a gate current “i”, so that the TRIAC Q1 is turned on.
On the other hand, a conventional phase controller shown in FIG. 22C is configured for flowing the gate current to the TRIAC Q1 from a predetermined phase angle to a zero-cross point of an AC voltage through a diode bridge DB by a transistor Q6, without using the DIAC or the SBS.
In these phase controllers, the TRIAC Q1 is turned on at a predetermined phase angle of the commercial power source AC so as to flow a load current of the incandescent lamp LA to the TRIAC Q1. Thus, when the phase angle is near to 90 degrees, a waveform of the load current shows precipitous rising up. Especially, a value of di/dt of the TRIAC Q1 at the turning on thereof is much larger, so that high frequency noise (150 kHz to 30 MHz) occurs. Furthermore, a filament of the incandescent lamp LA is vibrated due to the precipitous load current, so that acoustic noise further occurs.
For preventing the occurrence of the noises, in the conventional phase controller, a capacitor C0 (or a series circuit of capacitors C0 and C0′) is connected in parallel with the series circuit of the commercial power source AC and the incandescent lamp LA, and a reactance element L is connected between the series circuit of the commercial power source AC and the incandescent lamp LA and the TRIAC Q1, as shown in the figures. By such a configuration, the rising up of the load current becomes fluent so that the occurrence of the noises is reduced.
In the conventional phase controller, a very large reactance element L is necessary for reducing level of the noises satisfying a value of a standard such as IEC. Thus, the phase controller will be upsized. Furthermore, the reactance element L generates acoustic noise and/or heat. The phase controller used for large load shows such the tendency.
According to the Japanese Patent No. 2507848 or No. 2920771, a switching device such as a MOSFET, an IGBT (insulated gate bipolar transistor) or a power transistor is used as a phase control device instead of the TRIAC or thyristor.
Since a resistance of the switching device such as the MOSFET when the switching device is turned on is larger than that of the TRIAC, it is necessary to enlarge a radiator. Thus, the phase controller is upsized inevitably. Alternatively, for reducing the resistance of the switching device substantially equal to the resistance of the TRIAC, it is necessary to upsize the switching device itself. Thus, the phase controller becomes costly corresponding to upsizing the switching device.